3 ms·

1500 bytes is the de facto standard maximum size of an ethernet frame (not including headers). Therefore, if you want to send data over Ethernet, for example, a

1500 bytes is the de facto standard maximum size of an ethernet frame (not including headers). Therefore, if you want to send data over Ethernet, for example, an IP packet you can typically only send up to 1500 bytes at a time, and if you try to send more it gets dropped. This is called the Maximum Transmission Unit (MTU). Apparently, their customers were tunneling IP packets through another protocol, meaning that instead of sending an IP packet in a Ethernet frame, they were sending an IP packet in an X packet in an Ethernet frame. Since, like IP and Ethernet packets, the X packets need to contain some information related to the protocol, there was less room for the IP packet. I.e. the MTU was lower. When you set the MTU on your Operating System (OS), it refers to something slightly different. Instead of "this is the maximum size packet that will fit", it means "assume this is the maximum size packet that will fit". You can use that setting to force your OS to send smaller packets if you know the MTU is lower than your OS thinks.

zamadatix 5y ago
1500 payload bytes is not de facto, it's part of the actual 802.3 standard. Any other payload MTU is actually non-standard (and the IEEE has went out of their way to avoid standardizing other payload sizes. Frame size had a bump with 802.3as but still expressly left payload at 1500). MTU doesn't refer to anything different in that case, MTU always refers to the maximum size frame "this" node can handle. It also doesn't always mean the OS assumes it's the maximum size packet that will fit on a path it's the maximum size Ethernet frame the OS knows will fit on the NIC. The OS has other methods for assuming things about a path. Forcing MTU lower does force the OS to assume any path is never more than the MTU though which is why it works as a fix.

zamadatix 5y ago

1500 payload bytes is not de facto, it's part of the actual 802.3 standard. Any other payload MTU is actually non-standard (and the IEEE has went out of their way to avoid standardizing other payload sizes. Frame size had a bump with 802.3as but still expressly left payload at 1500). MTU doesn't refer to anything different in that case, MTU always refers to the maximum size frame "this" node can handle. It also doesn't always mean the OS assumes it's the maximum size packet that will fit on a path it's the maximum size Ethernet frame the OS knows will fit on the NIC. The OS has other methods for assuming things about a path. Forcing MTU lower does force the OS to assume any path is never more than the MTU though which is why it works as a fix.